The purpose of this work is to determine the effects of physical stresses on blood; specifically, we have demonstrated that shear stress as low as 300 dynes/cm to the 2nd power may result in decreased deformability of the erythrocytes. Such stresses may also cause platelet aggregation and release of platelet products. Shear stress of low magnitude has also been demonstrated to cause changes in leukocytes. These studies will help understand changes in the blood resultant from aberrant flow in vivo.